This invention relates to an optical element suitably usable in an exposure apparatus, a photographic system or an illumination system, for example, specifically in an optical system to be used with ultraviolet rays, visible light rays, or infrared rays and, more particularly, with deep ultraviolet rays or vacuum ultraviolet rays.
Conventionally, a diffraction grating of a certain type is used as a spectroscopic element in a spectroscope. Such diffraction grating has a grating shape of saw-teeth shape, and it is called a blazed type diffraction grating. Diffraction gratings of this type have a high diffraction efficiency which may be close to 100%. Recently, as an optical element using diffraction, binary optics (BO) elements having repetition of a very fine structure of step-like shape have attracted attentions. Particularly, among these elements, a lens called a BO lens and having an achromatic effect or an aspherical effect has attracted much attention because of a development possibility to a unique optical system.
A photographic lens (optical system) for a still camera, for example, to be used with visible light rays, can be produced by a molding method for molding a plastic or glass material by use of a metal mold. However, where a lens (optical system) can be used with light of short wavelength such as ultraviolet rays, use of a material having a ultraviolet-ray transmissibility as well as micro-processing works and a high machining precision are required. Currently, therefore, a molding method or lens materials adapted to this have not yet been developed. Further, the specification as required for a BO lens to be used with light of short wavelength such as ultraviolet rays is much beyond the machining limit (i.e., cutting precision limit) of a blazed type diffraction grating for a current spectroscopic element.
A BO (binary optics) lens to be used with ultraviolet rays or deep ultraviolet rays can be manufactured by using a quartz material. More specifically, an i-line stepper (xcex=365 nm) may be used for an exposure (printing) process, while a parallel flat pate type RIE apparatus may be used for a dry etching process. Thus, on the basis of a photolithographic technology and a dry etching technology, a diffractive optical element having repetitions of a very. fine step-like structure of eight levels (steps) can be produced.
When a high-energy laser beam such as ArF laser light (xcex=194 nm) or KrF laser light (xcex=248 nm) is projected on a quartz material, there occurs contraction. In consideration of it, a fluorite material has become attractive as a substrate material in place of the quartz material. Recently, development of photolithographic projection exposure apparatuses using exposure light in a vacuum ultraviolet region shorter than a wavelength xcex=200 has been desired. As regards a glass material Act constituting an optical system which enables use of such exposure light, the quartz material is insufficient in its performance. The fluorite material (fluoride compound) may be the only glass material that can be used in place of quartz. However, to a monocrystal fluorite material usable as a substrate material, it is difficult to execute a micro-processing operation based on a dry etching process.
It is an object of the present invention to provide an optical element having a very fine structure and being made by use of a fluoride compound such as fluorite, for example.
In accordance with an aspect of the present invention, there is provided an optical element, comprising: a substrate made of a monocrystal of a fluoride compound; and a fine structure formed on the substrate and made of a non-monocrystal of metal fluoride.
In accordance with another aspect of the present invention, there is provided an optical element, comprising: a substrate made of a monocrystal of a fluoride compound; and a fine structure formed on the substrate by forming, by deposition, a film of non-monocrystal of metal fluoride and by etching the metal fluoride film.
In these aspects of the present invention, the fine structure may be produced in accordance with a photolithographic method and a dry etching method.
The fluoride compound may contain at least one metal fluoride of calcium fluoride, magnesium fluoride, barium fluoride and aluminum fluoride.
A multilayered film comprising metal fluoride and alumina may be used.
A multilayered film comprising metal fluoride and calcium fluoride may be used.
The fine structure may comprise a thin film formed by using one of a sputtering method, an ion beam sputtering method, a CVD method and a vacuum deposition method.
The optical element may further comprise anti-reflection films formed on a top face and a bottom face of the substrate, respectively.
The fine structure may comprise a diffraction rating having a step-like sectional shape.
The fine structure may have one of a spherical surface and an aspherical surface.
In accordance with a further aspect of the present invention, there is provided an optical element, comprising: a substrate made of a monocrystal of a fluoride compound; and a fine structure formed on the substrate and made of a non-monocrystal of a fluoride compound.
In accordance with a yet further aspect of the present invention, there is provided an optical system having an optical element as recited above.
In accordance with a still further aspect of the present invention, there is provided an exposure apparatus including an optical system as recited above.
In accordance with a yet further aspect of the present invention, there is provided a device manufacturing method, comprising the steps of: exposing a workpiece with a device pattern by use of an exposure apparatus as recited above; and developing the exposed workpiece.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.